1. Field of the Invention
The present invention relates to an electrochromic display cell which produces or erases the color relying upon the electrochemical oxidizing/reducing reaction. More specifically, the invention relates to the construction of an electrochromic display cell which is capable of producing many displays.
2. Description of the Prior Art
In recent years, electrochromic display cells (hereinafter referred to as EC cells) have been developed as new display cells to substitute for the conventional liquid crystalline display cells, owing to their bright display, no limitation in the viewing angle and property to store the displayed contents. The EC cell, however, does not permit multiplex driving, but is driven by a current requiring electric charge of the order of several millicoulombs per square centimeter. Therefore, resistance of lead electrodes on a substrate of the EC cell must be very small. With the conventional EC cells, therefore, great limitation had been imposed with regard to the number of display electrodes when a plurality of displays are to be made or in regard to the arrangement of the display electrodes. Consequently, only very simple patterns could be displayed or, in other words, the displaying ability was poor.
FIG. 1 is a section view of a conventional EC cell having two layers that was proposed to eliminate the above-mentioned defect. Namely, a transparent upper substrate 11 and a transparent middle substrate 12 are adhered together via sealing members 14 to constitute a first cell, and the middle substrate and a lower substrate 13 are adhered together via sealing members 15 to constitute a second cell.
A transparent lead electrode 16 for a display electrode of the first layer is formed on the upper substrate 11, a display electrode 17 of the first layer composed of an electrochromic substance (hereinafter referred to as EC substance) is formed thereon, and an opposing electrode 19 of the first layer is formed on a transparent lead electrode 18 of the first layer that is formed on the opposing surface of the middle substrate 12. The opposing transparent lead electrode 18 of the first layer is electrically connected to an opposing lead electrode 21 of the first layer on the upper substrate 11 via an electrically conductive connection member 20.
The second cell is also constructed in the same manner. That is, a transparent lead electrode 22 for a display electrode of the second layer and a display electrode 23 of the second layer composed of the EC substance are formed on the opposite side of the intermediate substrate 12, and an opposing lead electrode 25 of the second layer and an opposing electrode 26 of the second layer are formed on the opposing surface of the lower substrate 13. The opposing lead electrode 25 of the second layer is electrically connected to an opposing lead electrode 28 of the second layer on the middle substrate 12 via an electrcially conductive connection member 27. Further, a reflection member 24 is disposed between the middle substrate 12 and the lower substrate 13.
The first and second cells are filled with electrolytes 29 and 30.
In the thus constructed multi-layer cell of the conventional art, the number of display patters can be increased by increasing the number of the layers, and the displaying ability can be increased correspondingly. This, however, causes the thickness of the whole cell to be increased; i.e., the cell becomes not suited for use as a display cell in the wrist watches that must be small in size and that must be thin. Further, the opposing electrode of the first cell must be transparent so that the display electrode of the second cell can be viewed. At present, however, there is available no transparent material which satisfies electrochemical characteristics required for the opposing electrode. There has been proposed a crystallized WO.sub.3 which, however, is not satisfactory in regard to the contrast of display, response speed and life. Moreover, the multi-layer cell is constructed through increased number of manufacturing steps. Consequently, the yield is poor and the manufacturing cost becomes high. Thus, the conventional EC cell of a multi-layer structure has many defects and can be used for particular applications only.